This invention relates to the field of solid state light emitting organic polymers
Light-emissive polymers are outstanding laser materials because they are intrinsically xe2x80x9c4-levelxe2x80x9d systems, they have luminescence efficiencies higher than 60% even in undiluted films, they emit at colors that span the visible spectrum and they can be processed into optical quality films by spin casting.
Since the discovery of laser action in polymers in 1992 [U.S. Pat. No. 5,237,582] remarkable progress has been made in implementing semiconducting polymer materials into different resonant structures for optically pumped lasers.[U.S. Pat. No. 5,881,083 and references therein; M. D. McGehee and A. J. Heeger, Adv. Mat. 2000, 12, 1 and references therein]. The high photo-luminescence quantum efficiencies of neat films with emission wavelengths ranging over the entire visible spectrum demonstrates the importance of this class of luminescent semiconducting polymers as gain media.
Early in 1996 Hide et al. [F. Hide, B. Schwartz, M. A. Diaz-Garcia, A. J. Heeger, Chem. Phys. Lett. 1996, 256, 424] observed lasing from polymers in the solid state for the first time when they blended titania nano-particles into a MEH-PPV/polystyrene film in these lasers, the random array of titania particles scattered the light emitted by the MEH-PPV in such a way that the feedback loops needed for lasing were provided. Later in 1996, four research groups independently observed stimulated emission from photopumped neat films of conjugated polymers. These observations showed for the first time that neat films, which were capable of conducting current, could in fact amplify light and that it was not unreasonable to attempt to make polymer diode lasers. Graupner et al. observed stimulated emission from films of a poly(para-phenylene)-type ladder polymer (using pump-probe techniques [W. Graupner, G. Leising, G. Lanzani, M. Nisoli, S. D. Silvestri, U. Scherf, Phys. Rev. Left. 1996, 76, 847]. Tessler et al. [N. Tessler, G. J. Denton, R. H. Friend, Nature 1996, 382, 695] obtained lasing by sandwiching poly(p-phenylenevinylene) (PPV) between a dielectric mirror and a silver mirror to form a microcavity. Hide et al. [F. Hide, M. Diaz-Garcia, B. Schwartz, M. Andersson, Q. Pei, A. Heeger, Science 1996, 273, 1833] and Frolov et al. [S. Frolov, M. Ozaki, W. Gellerman, V. Z., K. Yoshino, Jpn. J. Appl. Phys. 1996, 35, L1371; S. Frolov, W. Gellerman, M. Ozaki, K. Yoshino, Z. V. Vardeny, Phys. Rev. Left. 1997, 78, 729] observed line narrowing from films of PPV derivatives that were not part of a resonant structure. The mechanism of line narrowing was demonstrated to be from amplified spontaneous emission (ASE)[M. D. McGehee, R. Gupta, S. Veenstra, E. K. Miller, M. A. Diaz-Garcia, A. J. Heeger, Phys. Rev. B 1998, 58, 7035]. ASE occurs even when the gain coefficient is small because the spontaneously emitted photons are waveguided and thus travel a large distance through the gain medium, where they are amplified by stimulated emission.
In analogy with organic LEDs, one of the most obvious approaches to the injection laser is to use a vertical cavity laser configuration in which the active material is a thin film between two electrodes [N. T. Harrison, N. Tessler, C. J. Moss, K. Pichler, R. H. Friend, Opt. Mat. 1998, 9, 178; V. G. Kozlov, G. Parthasarathy, P. E. Burrows, V. B. Khalfin, J. Wang, S. Y. Chou, S. R. Forrest, IEEE J. Quant. Electr. 2000, 36, 18; M. A. Diaz-Garcia, F. Hide, B. J. Schwartz, M. D. McGehee, M. R. Andersson and A. J. Heeger, Appl. Phys. Lett, 70, 3191 (1997)]. Despite the fact that threshold current densities estimated from the excitation density required for optically pumped lasers have been exceeded in polymer diode structures by an order of magnitude [N. Tessler, N. T. Harrison, R. H. Friend, Adv. Mater 1998, 10, 64; I. H. Campbell, D. L. Smith, C. J. Neef, J. P. Ferraris, Appl. Phys. Lett. 1999, 75, 841] electrically pumped laser emission has not been demonstrated. The losses in the electrically pumped devices are higher than in simple photo-pumped waveguides because of two additional loss mechanisms: losses introduced by the metal electrodes and charge induced absorption [M. D. McGehee and A. J. Heeger, Adv. Mat. 2000, 12, 1].
The present invention provides a method for overcoming difficulties associated with the losses introduced by the metal electrodes and charge induced absorption by using an architecture known as the light-emitting field effect transistor (LEFET) configuration and to utilize injection-induced amplification of the xe2x80x9ccut-off modexe2x80x9d to achieve gain narrowing and lasing. In particular, solid state lasing structure is provided, comprising a field effect transistor in which source and drain electrodes are disposed on a semiconducting light emitting organic polymer forming an active layer on a gate whereby current between the source and drain electrodes defines and flows along a channel in the active layer to define a recombination and emission zone.
In a particular embodiment, a solid state lasing field effect transistor is formed of a solid, semiconducting light emitting organic polymer having a 4-level lasing energy system in which source and drain electrodes on one side and an indium-tin-oxide gate formed on the opposite side define the active layer containing the channel and recombination and emission zone. The gate is supported on a glass substrate and a SiO2 gate insulator layer is disposed between the gate and the light emitting organic polymer.
In a further embodiment, an additional layer of semiconducting organic polymer containing polycations and counteranions (or polyanions and countercations) is disposed between the source and drain electrodes and the light emitting organic polymer, and n and p doped regions are provided therein by applying a source-drain voltage at an elevated temperature for a time sufficient to mobilize the counteranions to form p-i-n junctions upon cooling, with an n doped region in contact with the source electrode and a p-doped region in contact with the drain electrode.
In another embodiment, Bragg or other reflectors are disposed on opposite sides of the channel to provide resonance with feedback whereby to generate coherent laser light.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.